This patent application is related to a concurrently-filed patent application entitled xe2x80x9cMethod of Measuring Anodize Coating Amount Using Infrared Absorbancexe2x80x9d and bearing attorney docket number BOEI-1-1049 and to a concurrently-filed patent application entitled xe2x80x9cMethod of Measuring Sol-Gel Coating Thickness Using Infrared Absorbancexe2x80x9d and bearing attorney docket number BOEI-1-1052, both of which are hereby incorporated by reference.
This invention relates generally to measuring amount and, more specifically to measuring inorganic coating amounts on metal surfaces.
When oxygen in the air comes into contact with aluminum, the oxygen and the aluminum are attracted to one another and form a natural layer on the aluminum surface. This oxide is made of a compound called aluminum oxide (Al2O3) and is chemically bonded to the aluminum, making it an integral part of the surface. If the oxide layer is scratched off, a new layer will re-form in the damaged area as soon as oxygen in the air contacts the aluminum. This natural oxide layer is 0.005-0.015 xcexcm thick and provides substantial corrosion protection.
A chromated conversion coating is a chromium oxide layer (Cr2O3) that is formed on aluminum in a solution containing water, chromates, and other chemicals, and that is around 100-200 times thicker than the natural oxide layer. The name xe2x80x9cconversion coatingxe2x80x9d is given to this type of coating because the natural aluminum oxide is converted to another type of oxide. For example, Alodine(copyright) 1200S is a chromated solution in which the aluminum oxide is converted to chromium oxide.
Chromated conversion coatings are created upon metallic substrates for a great variety of purposes. For example, chromated conversion coatings prevent corrosion and promote paint adhesion when applied to aluminum and other metal. Generally, a uniform coating amount or a coating amount within an acceptable range is required. However, determining uniformity of the coating amount or quantifying the coating amount relative to a desired range may be difficult. Current coating amount testing methods are destructive and therefore cannot be used with final production products. They are also time consuming, environmentally unfriendly, and disruptive to large scale production processes. Chromated conversion coating amount is sometimes specified for some applications and there is no simple non-destructive for measurement of chromated conversion coating amount currently known in the art.
Current coating amount testing known in the art is performed by measuring weight of a coated metallic substrate of known surface area. The coating is then chemically removed from the metallic substrate. The metallic substrate is reweighed and the difference is the weight of the chromated conversion coating, which is normally given in milligrams per square foot (mg/ft2). In addition, the currently known testing process only generates a spatially averaged coating amount for the sample. As such, the currently known testing process does not determine coating amount variations over an area.
Therefore there exists an unmet need in the art for a nondestructive method of determining chromated conversion coating amount on a metallic substrate.
The present invention provides a nondestructive method for efficiently determining the amount of a chromated conversion coating formed upon a metallic substrate without stripping the chromated conversion coating from the metallic substrate. The xe2x80x9camountxe2x80x9d of coating is suitably a coating thickness and, in a presently preferred embodiment, is preferably weight of the coating. The method may be employed in an in-line production facility or may be used intermittently as desired. The process may be used to provide a quantitative measurement, such as actual coating amount, or a qualitative measurement, such as a go or no-go result.
According to one embodiment of the invention, a non-destructive method is provided for determining the amount of a chromated conversion coating on a metallic substrate. A value of infrared energy reflected from the metallic substrate without the chromated conversion coating is determined. A value of infrared energy reflected from the metallic substrate with the chromated conversion coating is determined. A value of infrared energy absorbed in the chromated conversion coating is determined, and a value of the infrared energy absorbed in the chromated conversion coating is correlated to an amount of the chromated conversion coating.
According to an aspect of the invention, one embodiment of the invention includes transmitting an infrared beam having a predetermined wavelength through a chromated conversion coating on a metallic substrate at a predetermined incident beam angle. The transmitted beam has a cross-sectioned area to produce a predetermined spot size on a surface of the chromated conversion coating. The infrared beam is reflected off the metallic substrate to form a reflected beam and the reflected beam is filtered to a predetermined wavelength band, if desired, and detected. The infrared energy of the reflected beam is compared with a predetermined value of infrared energy reflected off the metallic substrate without the chromated conversion coating to determine an absorbance value for the chromated conversion coating. The absorbance value for the chromated conversion coating is correlated to an amount of the chromated conversion coating.